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Cisco ME 3400 Ethernet Access Switch Software Configuration Guide
78-17058-01
Chapter 30 Configuring QoS
Understanding QoS
Congestion Avoidance and Queuing
Congestion avoidance uses algorithms such as tail drop to control the number of packets entering the
queuing and scheduling stage to avoid congestion and network bottlenecks. The switch uses weighted
tail drop (WTD) to manage the queue sizes and provide a drop precedence for traffic classifications. You
set the size limits depending on the markings of the packets in the queue. Each packet that travels
through the switch can be assigned to a specific queue and threshold. For example, specific DSCP or
CoS values can be mapped to a specific egress queue and threshold.
WTD is implemented on traffic queues to manage the queue size and to provide drop precedences for
different traffic classifications. As a frame enters a particular queue, WTD uses the packet classification
to subject it to different thresholds. If the threshold is exceeded for that classification (the space available
in the destination queue is less than the size of the frame), the switch drops the frame.
Figure 30-7 shows an example of WTD operating on a queue of 1000 frames. Three drop percentages
are configured: 40 percent (400 frames), 60 percent (600 frames), and 100 percent (1000 frames). These
percentages mean that up to 400 frames can be queued at the 40-percent threshold, up to 600 frames at
the 60-percent threshold, and up to 1000 frames at the 100-percent threshold.
Figure 30-7 WTD and Queue Operation
In this example, CoS values 6 and 7 have a greater importance than the other CoS values, and they are
assigned to the 100-percent drop threshold (queue-full state). CoS values 4 and 5 are assigned to the
60-percent threshold, and CoS values 0 to 3 are assigned to the 40-percent threshold.
If the queue is already filled with 600 frames, and a new frame arrives containing CoS values 4 and 5,
the frame is subjected to the 60-percent threshold. When this frame is added to the queue, the threshold
would be exceeded, so the switch drops it.
WTD is configured by using the queue-limit policy-map class command. The command adjusts the
queue size (buffer size) associated with a particular class of traffic. You specify the threshold as the
number of packets, where each packet is a fixed unit of 256 bytes. You can specify different queue sizes
for different classes of traffic (CoS, DSCP, precedence, or QoS group) in the same queue. Setting a queue
limit establishes a drop threshold for the associated traffic when congestion occurs.
Note You cannot configure queue size by using the queue-limit policy map class command without first
configuring a scheduling action (bandwidth, shape average, or priority).
For more information, see the “Configuring Output Policy Maps with Class-Based-Weighted-Queuing”
section on page 30-44.
CoS 6-7
100%
60%
40%
1000
600
400
0
CoS 4-5
CoS 0-3
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